Microelectrode neural probes are an essential tool in neuroscience. They provide a direct electrical interface with the neurons of a biological entity's nervous system. Such neural probes can target the neuronal activity of neurons, enabling researchers and clinicians to better explore and understand neurological diseases, neural coding, neural modulations, and neural topologies. Moreover, the ability to analyze neuronal activity using neural probes has led to the development of new neuro-therapeutic devices implemented through brain-machine interfaces. These interfaces use neural probes implanted to bypass damaged tissue and stimulate neural activity, so that a patient can regain lost communication and/or control with respect to some aspect of the patient's nervous system.
One of the most recent types of neural probes are thin-film micromachined probes fabricated on silicon substrates using MEMS fabrication techniques. Signal recording sites of such silicon probes typically comprise exposed metal pads located on rigid silicon shanks that are connected, via interconnection traces, to output leads or to signal processing circuitry on a monolithic substrate. Silicon is the most widely used substrate for this type of microprobe because of its unique physical characteristics and widespread use in the microelectronics industry. These probes generally provide more control over the size and electrical properties of the recording and stimulating sites or drug delivery channels. Furthermore the silicon substrate allows integration of active circuitry that improves the quality of recording and stimulation applications as well as sensors, actuators, and even valves.
Despite the advantages of using a silicon substrate for neural probes, concerns exist about the mechanical strength of silicon substrate and its suitability for chronic biological applications due to the fact that bulk silicon substrate is a hard, fragile, brittle material and subject to breakage, especially during insertion where a silicon probe can break into several large or small pieces at the point of fracture. In case of a fracture, there is some risk that small pieces of silicon might remain in and damage the neural tissue or might migrate down into the brain. Even if the surgeon removes the body of the microprobe, he/she might not see all small fragments or may cause significant damage to the surrounding tissue if he/she tries to pull them out, since they usually have several sharp edges. Even if multiple insertions and removals are possible, the use of silicon may still remain a safety issue due to accumulated stress, fatigue, and microfractures from the prior insertions and removals.
Attempts to increase the mechanical strength of silicon probes have involved making the probe thicker and wider, which can also be problematic because of the possibility of severing or otherwise damaging the nerve tissue. In particular, the probe width and thickness cannot be increased to more than a few tens of microns due to risk of physical tissue damage. And another method of attempting to mitigate this risk of silicon breakage is to make the substrate of the microelectrode array flexible by utilizing thin-metal electrode sites and enclosing the wiring between polymer materials. The resulting electrode array is completely flexible, thereby providing needed strain relief. However, this design prevents direct insertion of the probe into brain tissue. Instead, with this type of probe, an incision much be first created to effect implantation. This typically results in increased tissue damage. Still other example approaches are disclosed in U.S. Pat. Pub. No. 2005/0107742 disclosing a shatter-resistant microprobe, and U.S. Pat. Pub. No. 2009/0299166 disclosing a MEMS flexible substrate neural probe.
What is needed is a microelectrode array probe that is sufficiently resistant to fracture and breakage into independent pieces upon insertion and implantation, especially one that is capable of withstanding multiple insertions and removals without buckling and breakage, i.e. having a buckling strength that is significantly greater than the force needed to penetrate that specific tissue and overcome the friction applied to the moving probe shank during insertion and removal. Furthermore, it would be advantageous to provide a microelectrode array probe capable of mitigating tissue damage during implantation, and that also can be relatively easily and efficiently fabricated in large numbers.